1. Field of the Invention
The present invention relates to a control system for a lock-up torque converter which includes a lock-up mechanism for automatic transmissions, and specifically to a system for controlling engagement and release of a lock-up clutch employed in the automotive lock-up torque converter.
2. Description of the Prior Disclosure
As is generally known, the conventional torque converter for automatic transmissions comprises a converter cover, a pump impeller integrally connected to the converter cover, a turbine runner firmly connected through a turbine hub to a transmission input shaft, and a stator provided between the pump impeller and the turbine runner to form a torque converter.
Recently, there have been proposed and developed various lock-up torque converters employing a lock-up clutch which is operable for directly and mechanically interconnecting the converter cover and the turbine runner to transmit torque from the engine crankshaft directly to the turbine runner during vehicle operation at speeds greater than a predetermined vehicle speed at which positive and negative fluctuations in torque transmitted from the crankshaft are minimal. One such lock-up torque converter has been disclosed in Japanese First Publication Tokkai (Showa) No. 63-172058. The conventional lock-up torque converter includes a control system for controlling engagement and release of the lock-up clutch. In the Japanese document No. 63-172058, the engaging force of the lock-up clutch is variably controlled by means of a duty-ratio controlled electromagnetic solenoid, such that the engaging force of the clutch is gradually decreased in accordance with a change in duty ratio of the solenoid.
The above-noted lock-up clutch is gradually released in accordance with the change in duty ratio imposed on the solenoid, even when the select lever is simultaneously shifted to an intended range position during releasing operation of the lock-up clutch. That is, the lock-up clutch is gradually released in response to the rate of change in duty ratio of the solenoid during the releasing operation, irrespective of the occurrence of shifting of the select lever. In the case where the lock-up clutch is merely released without shifting operation of the shift lever, it is preferable to gradually release the lock-up clutch depending on a change in duty ratio of the solenoid. However, in the case where the lock-up clutch is released simultaneously with shifting-up or shifting-down of the shift lever, there are some drawbacks, as follows.
Assuming that a change-speed instruction is generated just after generation of a releasing instruction for releasing the lock-up clutch, the change-speed operation may be performed before the lock-up clutch is completely released. As a result, the change-speed operation and the lock-up clutch releasing operation are partially overlapped. Therefore, so-called "select-shock" may take place. If the releasing operation of the lock-up clutch is always achieved rapidly, it may prevent the select shock. However, such a rapid release of the lock-up clutch itself results in a relatively large shock, even in the absence of the change-speed operation.